1. Field of the Invention
The present invention relates to a light guide configured to generate a plurality of linear light beams from a light beam emitted by a light source, and an illuminating device and an image reading apparatus using the light guide.
2. Description of Related Art
As examples of light guides of this kind, a light guide disclosed in Japanese Patent Laid-Open Publication No. 2014-187444 and a light guide disclosed in U.S. Pat. No. 8,279,499 are given. A light source is located to face an end of such a conventional light guide. A light beam entering from the light source is split by a beam splitter and directed into a plurality of rod-like light guiding sections arranged side by side with each other (arranged in the width direction). The light beam entering each of the rod-like light guiding sections propagates in the length direction of the light guiding sections while being total-reflected inside the light guiding section repeatedly.
At the bottom of each of the rod-like light guiding sections, a reflective section (for example, a group of prisms) is provided to reflect incident light upward. At the top of each of the rod-like light guiding sections, an exit surface is provided so as to be opposite the reflective section. The exit surface is designed so as to cause light entering from the reflective section, etc. to exit as a linear light beam such that a predetermined position can be irradiated with the linear light beam.
The above-described light guide is used in an image forming apparatus, for example. In this case, the light guide generates a plurality of linear light beams extending in the length direction of the light guide, and a document is illuminated with the linear light beams. Light reflected by the document reaches a linear image sensor, which is composed of CCDs (charged coupled devices) or the like, via a mirror and a lens. The quantity of light passing through a peripheral portion of a lens is lower than the quantity of light passing thorough a center portion of the lens. Therefore, the reflective section of each of the rod-like light guiding sections is designed so as to achieve a larger quantity of light at both end portions of each of the rod-like light guiding sections than at a center portion thereof with respect to the length direction.
In such a conventional light guide, each of the rod-like light guiding sections (that is, each portion capable of emitting a linear light beam) extends substantially from the forward end in the length direction of the beam splitter, and light emitted from a portion of the rod-like light guiding section near the beam splitter is influenced by the beam splitter. Therefore, in order to achieve a large quantity of light at the both ends in the length direction, the reflecting section must be located sufficiently far away from the beam splitter in the length direction. Thus, in the conventional light guide, each of the rod-like light guiding sections must be enlarged in the length direction in order to achieve a proper linear light beam.